Myocardial ischemia, and in severe cases acute myocardial infarction (AMI), can occur when there is inadequate blood circulation to the myocardium due to coronary artery disease. Evidence suggests that early reperfusion of blood into the heart, after removing a blockage to blood flow, reduces damage to the myocardium. However, the reestablishment of blood flow into the heart may cause a reperfusion injury to occur. Reperfusion injury is believed to be due to the build up of waste products on the myocardium during the time blood flow was inadequate and the reaction of these waste products with oxygen in the blood when normal blood flow is reestablished. It is possible to reduce reperfusion injury to the myocardium by cooling the myocardial tissue prior to reperfusion. Mild cooling of the myocardial tissue to a temperature between 28 and 36 degrees Celsius provides a protective effect, likely by the reduction in the rate of chemical reactions and the reduction of tissue activity and associated metabolic demands.
Local cooling is a site specific, temperature-reducing procedure that affects the cascade of events controlling the future health of the arterial wall that was recently damaged by a blockage in the blood stream. Emergency room procedures may include post-angioplasty local cooling of the lesion site. This additional procedure after dilating the lesion and re-opening the vessel is beneficial because clinical data has also shown that cooling the arterial wall just after angioplasty reduces restenosis or re-clogging of the artery. These outcomes can affect the long-term cost of treating the patient. However, short-term costs and ease of use are also important considerations.
Current technologies utilized in local cooling procedures vary widely. One method of cooling myocardial tissue is to place an ice pack over the patient's heart. Another method involves puncturing the pericardium and providing cooled fluid to a reservoir inserted into the pericardial space near the targeted myocardial tissue. Cooling of the myocardial tissue may also be accomplished by perfusing the target tissue with cooled solutions. Frequently, blood is taken from the angioplasty entry site (usually the groin), cooled outside the body, and then re-introduced into the patient, cooling the entire body. This approach is slow, due to requirements of cooling the whole body. In addition, the following re-elevation of the body temperature may require in excess of an hour. Cooling the blood requires a costly heat-exchanger, including the plumbing to transport blood from the patient to heat-exchanger and back. Cooling balloons present their own problems. Utilization of this cooling technology requires a cold flow of inflation media to the balloon and back. This is accomplished using complicated, multiple lumen catheter shafts. An external cold media is slowly pumped through the catheter at a predetermined flow rate, to the dilating pressure level. This is neither a simple nor low cost task. In addition, with perfusing balloons, the perfusion rate is often so high, that without a very large orifice to deliver the cold media, jetting of the media can occur and put the vessel at risk for further damage.
Direct injection of cold media often has little impact. Without the capability to hold the temperature at the desired target for an extended period, there is often no effect or benefit from the injection of cold media. Although injection of large amounts of cold media can extend the temperature reduction, this may lead to additional complications. When the flow of cold media is stopped, the arterial branch infused with the media may be shocked by the change. This can cause spasm or other reactions, and damage to the vessel.